Thermal flattening silicon steel strip has required many more considerations than just flatness. In the past, electrical steel has had to consider the influences of the thermal flattening on annealing separators, secondary coatings and magnetic properties. Thermal flattening has also been combined with stress relief annealing as part of the same process. Silicon steel strip which is produced in the flattened condition but requires additional heat treatment is identified as semi-processed and the customer typically provides a stress relief anneal after the electrical steel sheet is fabricated into laminations which are then assembled into the electromagnetic equipment.
In the manufacture of wound core transformers, the electrical steel is subjected to severe mechanical stresses during fabrication of the core which ultimately must possess excellent magnetic properties. The magnetic properties are developed after a stress relief anneal at temperatures of at least about 1450.degree. to 1500.degree. F. (785.degree. to 815.degree. C.). Grain oriented electrical steels are particularly suited for use in transformers with wound cores. This equipment requires excellent flatness in the strip.
Strains are developed in silicon steel production from numerous operations and conditions. If the strains are not removed, there is an increase in hysteresis loss when the steel is used in electrical equipment and this impairs its magnetic properties. The strains from slitting, winding and fabricating during core production must also be removed to achieve the desired magnetic properties.
Oriented electrical steel is thermally flattened to produce strip for transformers or generator laminations. Flattening the strip involves the use of tension to remove irregularities such as deformed edges, wavy edges, and buckles. However, the use of tension and the resultant elongation introduces stress which needs to be minimized. Temperatures of about 815.degree. C. (1500.degree. F.) are frequently used during flattening to remove the stresses caused by flattening and prior processing.
The tension limitations for electrical steel at elevated temperatures have been the subject of many investigations. U.S. Pat. No. 2,351,922 describes the strip tension of 500 to 2000 pounds per square inch which is below the elastic limit of the alloy. The temperatures during tension were from 700.degree. C. to 825.degree. C. for periods of about 1 to 2 minutes.
U.S. Pat. No. 2,412,041 taught the tensile strength of the electrical steel varies with the temperature and that a tension sufficient to prevent sagging between the support rolls will produce excellent flatness in the strip. This tension is provided by operating the exit rolls at a peripheral speed of 0.1 to 0.5% faster than the entrance rolls. The amount of tension required will vary with the composition and the gage of the material, the temperature and the length of time. The patent states a permanent elongation of 0.15 to 0.3% is normal. Temperatures as low as 1200.degree. F. are mentioned but flatness control was only required to be below the graphite solubility temperature. If the carbon content is low, much higher temperatures may be used and the limit is determined by the mechanical factors. An electrical steel with less than 4% silicon which has been decarburized may be annealed at around 1500.degree. to 2100.degree. F. under tension to produce the desired magnetic properties and flatness. If the material is brought to the softening temperature under tension, there is not much of a restriction on holding time. The strip will reach the desired temperature within about 1 minute, depending on gage, and is preferably cooled slowly. Atmosphere forms no limitation on the invention since it does not affect flatness. The atmosphere is selected in accordance with its effect on core loss, ductility or brightness.
U.S. Pat. No. 3,130,088 describes the influence of roll diameter during flattening and the spacing between the rolls. Part of the furnace relies on a series of rolls which alternately pass the strip over and under the rolls to increase the flatness. The best results were obtained by using a preferred temperature of 1450.degree. F. to 1500.degree. F. with 1100 to 2200 psi strip tension. The patent admits it is impossible to describe the combination of stresses which produce the flattening at elevated temperatures. The phenomenon of creep and structural instability of metals at elevated temperatures made the process too complex because of the interplay of the many variables.
U.S. Pat. No. 3,161,225 attempted to flatten the electrical steel strip without introducing any stress to provide the optimum magnetic properties. A controlled reverse curvature of the strip was found to remove coil set during flattening and minimize the stress caused by tension and flexing of the strip. It was taught that a plastic strain as small as 0.05% elongation caused by bending or tension resulted in irrecoverable damage to the magnetic properties. Tension is limited to be no greater than that required to advance the strip. Particularly this level should be below 1000 psi and preferably about 100 psi.
Prior thermal flattening processes for electrical steel have thus known there is a wide range of conditions which produce a flat strip. However, the flattening process has typically been one which minimizes stress and thus uses low tension for low stress or uses high temperature for flattening which is part of a stress relief anneal. The prior work done with various thermal flattening processes have ignored the influence of the conditions on the coatings. The coatings were expected to survive or be modified to not require any special considerations.
The prior practices for thermal flattening grain oriented silicon steel have varied considerably. Tension has varied from 100 psi up to the elastic limit of the steel. Temperatures from 900.degree. F. to 2100.degree. F. have been investigated. Various roll configurations and diameters have been studied. However, the prior studies have not taken into consideration the influence of the flattening conditions on the responsiveness of the material to a stress relief anneal. Prior processes have been mainly directed to the fully processed material and have not found the conditions for flattening which are most responsive to stress relief annealing by the customer after the electrical steel products are fabricated.
Prior practices have not investigated what the temperatures and tensions were doing to the surface coatings. The combination of thermal flattening and stress relief annealing for semi-processed silicon steel is basically a rather new product for oriented silicon steel.
Thinner gages of electrical steel have considerably more problems in wound core applications than prior material of heavier gages but the improved magnetic properties justify their use. With thinner material, there is more difficulty in gage control, there is less stiffness in the material, there is more difficulty in obtaining the desired flatness and there is more of a winding or handling problem because of coil set and shape problems.
It is a principle object of the present invention to develop a practice for thermal flattening silicon steel which optimizes the magnetic quality of the steel for wound core applications and other semi-processed applications which require stress relief annealed after fabrication. A further principle object of the present invention is to improve the tension imparting characteristics of a secondary coating by modifying the conditions of the thermal flattening process.
Another object of the present invention is the development of a process which uses moderately low temperatures and higher tension to provide thermal flattening for wound transformer core applications. This has considerable advantages over other practices where extremely low levels of tension are used in this processing step. The present invention allows high tension levels which improve strip tracking in the furnace. The present invention also increases the yield strength of the base metal during thermal flattening to allow the use of high tension without damage to the base metal at the elevated temperatures. The present invention also permits the use of furnaces for thermal flattening which previously could not be used because of tension limitations.
A further object of the present invention is to provide a semi-processed silicon steel strip which after the thermal flattening process of the invention will provide improved handling characteristics during winding of the core and improved magnetic properties after the stress relief anneal.
The thermal flattening process of the present invention provides other advantages which will become apparent to those skilled in the art from the description which follows.